1. Field of the Invention
The present invention relates generally to ocular surgical procedures involving implantable or injectable lenses into the eye of a recipient, and more specifically to systems and methods for determination or selection of a lens power for providing emmetropic vision or, if chosen, a specific ametropic vision to a pseudophakic eye.
2. Description of the Related Art
Accurate determination of lens power is an important aspect in providing emmetropia, or a′ desired degree of ametropia, to a subject undergoing cataract surgery or other ophthalmic procedures in which the natural crystalline lens is replaced with or supplemented by implantation of an intraocular lens (IOL) into the eye. Measurements of the eye are typically made preoperatively and a lens power is selected based on correlations between the measured values and lens powers providing a desired refractive outcome.
Over the years a number of intraocular lens power calculation formulas have been developed, for example, as discussed in the book published by SLACK Incorporated entitled Intraocular Lens Power Calculations, by H. John Shammas, which is herein incorporated by reference in its entirety. These power formulas may be broadly characterized into at least two categories: theoretical formulas, which are based on a geometric optic, two-lens vergence formula; and regression formulas, which are based on regression formulas obtained by fitting data from a large patient database to an equation relating lens power to one or more parameters thought to correlate with lens power. While there has been continued progress in the accuracy of intraocular lens power calculation formulas to obtain better refractive outcomes, undesirable refractive outcomes due to improper intraocular lens power calculations are still relatively common. Apart from the general desire for spectacle-free refractive outcomes, demands for more accurate lens power calculation have also increased due to the introduction of multifocal lenses.
Many of the current formula algorithms were derived by optical back-calculation to agree with the refractive outcome. In this manner they may be confounded with errors in all parameters used in the calculation, and the oversimplification of thin-lens theory. An evaluation of the sources of errors in lens power calculations was recently published by one of the current co-inventors (Sverker Norrby, “Sources of error in intraocular lens power calculation”, Journal of Cataract and Refractive Surgery, Vol. 34, pp. 368-376, March 2008, which is herein incorporated by reference in its entirety). In this paper, preoperative estimation of postoperative intraocular lens position was determined to be the largest contributor of error in the refractive outcome of cataract surgery, with an error contribution of 35%, relative to all error sources evaluated.
In most, if not all of the current formula algorithms, there are a number of ocular parameters that are used in deriving an appropriate lens power for implantation into the eye. These parameters include axial length (AL), corneal radius (CR) or power (K), and anterior chamber depth of the natural crystalline lens prior to surgery (ACDpre). In general, one or more of these parameters are used to provide the preoperative estimation of postoperative intraocular lens position. The estimated postoperative lens position is then used in combination with one or more of these same parameters to provide an estimate of the correct lens power to provide a desired refractive outcome (typically emmetropia). However, as discussed in the previous paragraph, the use of this term in calculating postoperative lens position is a large error source in this process. In addition, some of these parameters may be unavailable at the time of evaluation. For example, in the case of a patient that has previously received a corneal refractive surgery, such as LASIK or PRK, the original corneal radius or power may no longer be available. It is the corneal radius prior to corneal refractive surgery that is correct to use in the power calculation formulas because they were developed for normal eyes on data pertaining to normal eyes. The corneal refractive surgery has changed the anatomic relations of ocular dimensions. Hence, the CR or K determined for corneas that have had corneal refractive surgery will give erroneous estimates of the IOL position.
Accordingly, better systems and methods are needed that will allow reliable and accurate determination of implanted or injected lens power, and to provide such determination even in cases where knowledge of information such as original corneal radius or power is no longer available.